High frequency amplifier circuit

Abstract

A high frequency amplifier circuit based on an amplifying transistor, e.g. a dual-gate FET, having an A.G.C. voltage applied to a control terminal thereof to control the transistor DC operating current, has a diode connected between an input electrode of the transistor and a source of an input high frequency signal. All or part of the DC operating current of the transistor is passed through the diode as the operating current of the diode to thereby produce a high degree of attenuation by the diode when the transistor current level is small, so that reduced cross modulation interference is produced by the transistor when a high degree of gain reduction is executed by A.G.C. control.

What is claimed is: 
     
       1. A high frequency amplifier circuit comprising: a transistor having an input electrode and having a gain control electrode which is coupled to receive a gain control DC voltage for controlling a DC operating current of said transistor to thereby control a degree of gain of said transistor in accordance with the level of said DC operating current;   a diode coupled between said input electrode of said transistor and a source of an input high frequency signal, for transferring said input signal to said transistor input electrode while applying a degree of attenuation to said input signal during said transfer which is determined in accordance with a level of a DC operating current of said diode; and   high frequency blocking means coupled to pass at least a part of said DC operating current of said transistor through said diode as said DC operating current of said diode.   
     
     
       2. A high frequency amplifier circuit according to claim 1, in which said high frequency blocking means comprise at least one choke coil connected between an electrode of said transistor and said diode. 
     
     
       3. A high frequency amplifier circuit according to claim 1, in which said transistor is a dual-gate field effect transistor having a first gate electrode which functions as said input electrode, and a second gate electrode which functions as said control electrode. 
     
     
       4. A high frequency amplifier circuit according to claim 3, in which at least a part of a source current of said field effect transistor flows through said diode. 
     
     
       5. A high frequency amplifier circuit according to claim 3, in which at least a part of a drain current of said field effect transistor flows through said diode. 
     
     
       6. A high frequency amplifier circuit according to claim 1, in which commencement of reduction of gain of said high frequency amplifier circuit by attenuation action of said diode is delayed with respect to a reduction of gain of said high frequency amplifier circuit resulting from gain control action of said transistor.